Yep, this is by design. This is why they are included in the gray color after the append ‘+’, which indicates that they are not to be included in your reviews today.
The ‘+X’ designation indicates that the cards are suspended, which is related to your deck settings, specifically sibling burial:
Not quite, since those cards are buried it means that they won’t show up today at all. They will likely show up tomorrow, unless they are buried again.
You can adjust your deck settings per the image I shared above which will control which, if any, cards are buried. But, it’s not related to FSRS either way so not much to adjust there.
Please let me know if that answers your question, and if there’s anything else I can help with. 